Ultrasound may be used in a variety of medical applications, including diagnostic ultrasonography of the eye. Diagnostic information is typically provided by an ultrasound pulse from a piezoelectric transducer, which is directed into a tissue. Reflected acoustic energy is detected (as ‘echoes’), so that the amplitude of the received energy may be correlated with the time delay in receipt of the echo. The amplitude of the echo signal is proportional to the scattering strength of the refractors in the tissue, and the time delay is proportional to the range of the refractors from the transducer. A variety of hand-held ultrasound instruments for measuring corneal thickness (called pachymeters) have been developed (for example see U.S. Pat. Nos. 4,564,018; 4,817,432; 4,930,512). Many prior art ultrasonic pachymeters provide A-scan output, in the form of waveforms displayed on a cathode ray tube, representing acoustic reflections in a single dimensional ‘column’ of tissue.
In B-scan ultrasonography, a two-dimensional image is formed, in which pixel brightness reflects the amplitude of the reflected acoustic signal. A B-scan image therefore represents a cross-sectional slice of the imaged tissue. The cross-sectional information is typically provided by correlating information from a series of adjoining columnar scans (each of which may be used to produce A-scan output). For the purpose of producing B-scans, adjoining columnar scans may be produced by a number of methods: rectilinear translocation of a transducer over the tissue of interest; pivoting angular displacement of a single transducer over a fan-shaped area; or through the use of a linear array of transducers.
In some applications, three dimensional images may be reconstructed from a series of B-scans. U.S. Pat. No. 4,932,414 to Coleman et al. for example describes a system in which the transducer is electronically swept or physically rotated to produce a series of sectored (fan-shaped) scan planes which are separated by a known angular distance, to produce a 3-dimensional display. In a similar fashion, U.S. Pat. No. 5,487,388 to Rello et al. discloses an ultrasonic scanning system in which sequential fan-shaped B-scan image planes are obtained by movement of the transducer probe in an arc, a movement which allows the apex of the scanned 3-dimensional volume to be located below the probe to facilitate imaging between closely-spaced surface obstructions.
The structure of the eye, particularly the cornea, presents special problems for optimal ultrasonographic B-scan imaging. The human cornea is an asphere, flattening concentrically, typically approximately 11 mm across with an average central radius of curvature of 7.8 mm which increases towards the periphery. The high resolution required for ultrasonic imaging of some corneal structures is optimally achieved if ultrasound data is collected from the focal point of the transducer, and the ultrasound beam is normal to the surface of the cornea. As a result, rectilinear scanning of the cornea provides optimal imaging information only from relatively small segments of the cornea which are normal to the transducer beam and in the plane of beam focus. Similarly, volumetric 3-dimensional scanning by reconstruction of a series of fan-shaped B-scan planes, as for example described in U.S. Pat. Nos. 4,932,414 and 5,487,388, is not a system adapted to provide the degree of resolution required for biometry of the corneal surface.
High frequency ultrasound has been used in ophthalmological ultrasonography to obtain biometric B-scan images of the human cornea, by arcuate translocation of a single element focused transducer. Silverman et al., 1997, J. Ultrasound Med. 16:117-124, describe a system for sonographic imaging and biometry of the cornea in which a sophisticated programmable motion system permits ultrasonographic arc scanning. In the Silverman et al. system, the ultrasonic transducer is translated through an arc matched to the approximate radius of curvature of the cornea using five servo motors and a controller. Similarly, U.S. Pat. No. 5,331,962 discloses an ultrasound system for corneal arc scanning, in which a transducer is translocated along a curved track that approximates the surface curvature of the cornea.